In motor vehicles, ultrasound sensors are used as part of parking assistances, for measuring parking gaps or for illuminating blind spots. These sensors emit sound pulses and pick up signals reflected by obstacles, amplify them, convert them to evaluable signals and supply them to a control unit. In addition, the sensors have a sound transducer and corresponding transmit and receive electronics. A pot-shaped sound transducer has proven itself as a particularly robust construction, whose pot floor is formed by a resonant diaphragm that is excited by a piezo element. In order to make possible operating ranges of the sensor of approximately 3 m, the piezo element is activated using a comparatively high voltage between 50 Volt and 100 Volt, which is normally generated by a transformer or a transmitter, as the case may be, having a transformation ratio greater than 1. On the side of the primary winding, the primary side, a d.c. voltage is switched on and off periodically, in this context, whereby on the secondary side, the side of the secondary winding, a higher voltage may be picked off, corresponding to the transformation ratio. This is used as the activating voltage for the sound transducer.
The echo signals reflected by the obstacles typically have amplitudes between 0.1 mV and 1.0 mV. There are, however, interference sources in the surroundings of the vehicle and in the vehicle itself, which produce interference signals that are also in the working frequency of the sensors and that have comparable amplitudes. Interference comes especially frequently from control units of fluorescent strip lamps and control units of gas discharge lamps in automobile headlights. Since the input of the amplifier is the most sensitive place of the evaluation electronics, these interferences are coupled in via the amplification path.
The following measures are conventional for suppressing interference: First, low pass, high pass and band pass filters are provided in the circuit topology and the dimensioning of the circuit. In addition, it is known that one may develop the layout of the circuit suitable for the EMC (electromagnetic compatibility) and in multilayer. One may also work using screening of the housing and of parts of the circuit.
DE 102 48 677 describes a method for the compensation of magnetic interference fields. Since the transmitter itself functions as the antenna for receiving the magnetic interferences, DE 102 48 677 proposes using a second transformer having the identical properties.